I would venture to guess that your path to 'autoload.php' is wrong. Looks like the 'require' is looking for 'autoload.php' in a folder called 'vendor' inside the current folder of 'C:\xampp\htdocs\sample-code-php-master\PaymentTransactions\'. If the 'vendor' folder is not inside (under) that folder then you need to fix the 'require' path.